And The 121 Dollar Word Is!

Idempotent, which is an adjective that describes something that has the same effect no matter how many times you do it. Which is something very impotent to achieve in clean up functions. Because when using manual resource management it can often be nearly impossible to tell whether something has been released or not at any one particular point in the code. So programmers tend to "over release" things to be safe. That is they release things again because they are not sure if it has been done yet or not.

This is much less of a problem in object oriented languages because you can use RAII. But in classic C where I spend most of my day we don't have destructors so that technique is not helpful. For some reason people sometimes write clean up functions that cannot be run twice without problems. Which inevitably causes problems. Because of what I described before. When writing any such function you should very carefully make sure that if it is run on an already released resource can detect that case and do nothing.